1. Field of the Described Embodiments
The described embodiments relate generally to display devices. In particular, apparatus, method and system for providing an ambient light calibration factor used in a transmissive display are described.
2. Description of the Related Art
Solid state displays that use solid state elements such as liquid crystal, or LC, for presenting visual content have become ubiquitous. In a particular type of solid state display, a light source, referred to as a backlight, provides illumination that is used to form an image on a viewable display panel. For example, in those solid state displays that utilize liquid crystal image elements (referred to as a liquid crystal display, or LCD), the backlight can take the form of a discrete light source. In some cases, the backlight can take the form of a plurality of light emitting diodes, or LEDs, that can provide a substantially white light. The white light, in turn, that can be projected through an image forming layer having a plurality of image elements. The plurality of image elements can include a liquid crystal material that can be selectively rendered almost fully transparent to almost fully opaque based upon an image signal applied to control elements. When combined with color filters (usually three color filters are used representing the primary colors, red (R), blue (B), and green (G)), the plurality of image elements can form an array of pixels that can be used to create an image that can be viewed on a display panel that is typically covered by a protective layer formed of glass or plastic.
However, in order to provide a viewer with an acceptable (or in some cases, exceptional) viewing experience, the viewable image should appear bright and not washed out under all ambient light conditions. For example, in a viewing area that is brightly lit (naturally by sunlight or artificially using, for example, incandescent lighting), the image presented on the display panel can appear washed out due to the high ambient light level reducing the overall contrast between the displayed image and the surrounding area. Therefore, a number of displays attempt to maintain an acceptable viewing experience by using an ambient light sensor to detect an ambient light level. The ambient light level is then used to adjust the light output of the backlight. For example, the ambient light sensor compensates for ambient light by making the display bright enough for an acceptable viewing experience. Therefore, it is important for optimal viewing and power consumption that any change in ambient light level detected by the ambient light sensor be effectively compensated by modifying the amount of light provided by the backlight. This is particularly true for energy efficient display systems since it is the backlight that consumes a substantial amount of the power required to operate the display. Unfortunately, however, the optical path of a display system can include several optically active layers through with ambient light must pass before being detected by the ambient light sensor. Each optically active layer can contribute to an overall optical path tolerance, or variation. This variation can be on the order of ±80% indicating that an ambient light level L1 detected by the ambient light sensor can only be correlated to an actual ambient light level in the range of 0.2 L1 to 1.8 L1 making efficient backlight control difficult. Moreover, this large variance can result in a concomitantly large variance in display screen luminance.
In order to qualify as energy efficient (Energy Star, for example), a consumer product, such as a display, must meet certain requirements for power use and efficiency. Since the backlight typically accounts for most of the energy used by the display, it is important to be able to efficiently and effectively control the power used by the backlight in order to meet a specific energy standard. Unfortunately, since the optical path tolerance makes effective and efficient ambient light control of the backlight difficult to achieve, display manufacturers compensate by reducing the overall light output of the backlight for all ambient light conditions. This reduction in overall light output typically results in an inferior image presented by the display.
In view of the foregoing, there is a need for providing an energy efficient display that provides a viewer with a desirable viewing experience under most if not all ambient light conditions.